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(#) Missing Android XML namespace

!!! ERROR: Missing Android XML namespace
   This is an error.

Id
:   `MissingPrefix`
Summary
:   Missing Android XML namespace
Severity
:   Error
Category
:   Correctness
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   7.0.0 (July 2021)
Affects
:   Manifest files and resource files
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/MissingPrefixDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/MissingPrefixDetectorTest.kt)

Most Android views have attributes in the Android namespace. When
referencing these attributes you **must** include the namespace prefix,
or your attribute will be interpreted by `aapt` as just a custom
attribute.

Similarly, in manifest files, nearly all attributes should be in the
`android:` namespace.

!!! Tip
   This lint check has an associated quickfix available in the IDE.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
res/layout/namespace.xml:2:Error: Attribute is missing the Android
namespace prefix [MissingPrefix]
&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:other="http://foo.bar" android:id="@+id/newlinear" android:orientation="vertical" android:layout_width="match_parent" android:layout_height="match_parent" orientation="true"&gt;
                                                                                                                                                                                                                                          ------------------
res/layout/namespace.xml:3:Error: Attribute is missing the Android
namespace prefix [MissingPrefix]
    &lt;Button style="@style/setupWizardOuterFrame" android.text="Button" android:id="@+id/button1" android:layout_width="wrap_content" android:layout_height="wrap_content"/&gt;
                                                 ---------------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`res/layout/namespace.xml`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~xml linenumbers

&lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:other="http://foo.bar" android:id="@+id/newlinear" android:orientation="vertical" android:layout_width="match_parent" android:layout_height="match_parent" orientation="true"&gt;
    &lt;Button style="@style/setupWizardOuterFrame" android.text="Button" android:id="@+id/button1" android:layout_width="wrap_content" android:layout_height="wrap_content"/&gt;
    &lt;ImageView android:style="@style/bogus" android:id="@+id/android_logo" android:layout_width="wrap_content" android:layout_height="wrap_content" android:src="@drawable/android_button" android:focusable="false" android:clickable="false" android:layout_weight="1.0" /&gt;
    &lt;LinearLayout other:orientation="horizontal"/&gt;
&lt;/LinearLayout&gt;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/MissingPrefixDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `MissingPrefixDetector.testBasic`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Adding the suppression attribute `tools:ignore="MissingPrefix"` on
  the problematic XML element (or one of its enclosing elements). You
  may also need to add the following namespace declaration on the root
  element in the XML file if it's not already there:
  `xmlns:tools="http://schemas.android.com/tools"`.

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="MissingPrefix" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'MissingPrefix'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore MissingPrefix ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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